Passive margin

A portion of the mid-Atlantic ridge above the
sea surface in Iceland.
Chapter 2
The Sea Floor
What does geology have to do
with marine biology?
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Before we start:
Marine biology =
Geology =
Geo- , Latin for__________
The study of earth's
physical structure and
substance, its history, and
the processes that act on it.
The Water Planet
The Geography of the Ocean Basins
The oceans cover ____of the
planet and regulate its
___________ and ___________
There are four ocean basins
 Pacific Atlantic
 Indian
 Arctic –
 Connected to the main
ocean basins are shallow seas
Mediterranean Sea, Gulf of
Mexico, South China Sea
Fig. 2.1
The Geography of the Ocean Basins
 They all connect to form
a world ocean where
seawater, materials, and
organisms can move
 Continuous body of
water surrounding
Antarctica is the
Southern Ocean
Fig. 2.2
The Structure of Earth
 Big Bang Theory
 Universe created 13.7
billion years ago.
 Earth formed 4.5 billion
years ago.
Fig. 2.2
The Structure of Earth
 In the early molten Earth,
lighter materials floated
toward the surface because
of varying densities.
 They cooled to form the
 The atmosphere and
oceans then formed
 Earth is the right distance
from the sun for liquid
water, and life, to exist
 Exists only on Earth, due to marrow temp. range
required for liquid water.
 Earth is in prime position/size for liquid water
 Any closer-too hot, water evaporates
 Any further-too cold, all water freezes
 Any smaller (~30%) then not enough gravitational pull
to keep water on surface, all moves to atmosphere.
 Any bigger, then too many clouds
-Importance of clouds? Greenhouse Effect
-more clouds = warmer surface (Ex: Venus)
How did the water form?
 As Earth cooled, water
vapor in atmosphere
condensed, fell to
surface, filled up low
parts first.
Fig. 2.3
The Structure of Earth
Internal Structure
 The dense CORE is mostly iron
 Solid inner core and liquid outer core
 Believed the swirling motions produce the Earth’s
magnetic field
 The mantle is outside the core and under the crust
 Near molten rock slowly flows like a liquid
 The crust is the outer layer, comparatively thin
 Like a skin floating on the mantle
 composition differs between oceans and continents
The Structure of Earth
Continental and Oceanic Crusts
Oceanic Crust
Continental Crust
 Made of basalt – a dark
 Made of granite – lighter
 More dense
 Thinner
 Younger rock; 200 mil years
 Less dense
 Thicker
 Older rock; ~4 bil years ago
Tab. 2.2
The Structure of Earth
Continental and Oceanic Crusts
 Continental crust floats higher on the mantle and
ocean crust floats lower.
 That’s why ocean crust is covered by water
Fig. 2.2
The Origin and Structure of the Ocean Basins
 Earth today: still dynamic
 Not static and unchanging
 Continents still moving!
 Size/Shape of ocean basins defined by continental
You should be asking, where is your evidence that continents move!?
Alfred Wegener
 Looked at a map of
Earth, continents look
like puzzle pieces.
Alfred Wegener
 1st to come up with theory about
continental drift 1912.
 Suggested that all the continents
had once been a supercontinent,
named Pangea
 He thought, started breaking up
~180 mil years ago
 Continents as
puzzle pieces, S.
America and Africa
 Other evidence:
 Similar rock
 Fossil records
The Theory of Plate Tectonics
 The Theory of Plate
Tectonics explains it all
 Continents do drift
slowly around the world
 This process involves
surface of the entire
The Theory of Plate Tectonics
Discovery of Mid-Ocean Ridge
 After WWII sonar allowed detailed maps of the sea
 SONAR = sound navigation ranging
 They discovered the mid-ocean ridge system!
 Chains of ridges in the middle of the oceans, like
seams on a baseball
 The largest geological feature on Earth
Fig. 2.5
Mid-Ocean Ridge System
 Some of the mountains rise above sea level to form
islands, e.g. Iceland
 The Mid-Atlantic ridge runs down the center of the
Atlantic Ocean and follows the curve of the opposing
 Sonar also discovered deep trenches = deep
depressions in the sea floor.
Fig. 2.6
Significance of the Mid-Ocean Ridge
 Rock near the ridge is young and gets older moving away
from the ridge
Significance of the Mid-Ocean Ridge
 There is little sediment
near the ridge, but it gets
thicker moving away
 Sediment = loose material
like sand and mud that
settles on bottom of sea
 Found symmetric
magnetic bands parallel to
the ridge where magnetic
field flip-flopped
Magnetic Anomalies
 Earth’s magnetic field, occasionally reverses direction
 Magnetic parts in molten rock, free to move.
 When cool, these particles are “frozen” and keep their
orientation, even if magnetic field changes.
 Sea floor rocks have these bands, or magnetic
 ****Sea floor NOT formed all at once****
 Offered a mechanism for movement of continents
Fig. 2.9
Creation of
Sea Floor:
Sea Floor
Cross section of the sea
floor at a mid-ocean ridge.
The rocks of the sea floor
show the earth’s
at the time of their
Creation of Sea Floor: Sea-Floor
 Huge pieces of oceanic crust are separating at the mid
ocean ridges
Magma from the mantle rises through the rift (a crack)
forming the ridge
The sea floor moves away from the ridge
Continuous process, called sea-floor spreading
New sea floor is created!
This explains why rocks are older and sediment is thicker
as you move away from the ridge
This also explains the magnetic stripes found in the sea
Sea-Floor Spreading
and Plate Tectonics
 The crust and part of the upper mantle form the
 100 km (60 mi) thick, rigid
 Broken into plates
 May be ocean crust, continent crust, or both
 The plates float on a fluid layer of the upper mantle
called the asthenosphere.
•Edge of many plates, a mid-ocean ridge
• The plates move apart here, to create new sea floor
• If the plate has continental crust it carries the
continent with it.
• Spread 2-18 cm/year
• Called continental drift, continents moving apart
Plate Boundaries at Trenches
 If seas floors are spreading, does this mean the Earth is
growing bigger?
 No it means that somewhere else plates get pushed down into
the mantle
 As new lithosphere is created, old lithosphere is destroyed
somewhere else
 Some plate boundaries are trenches where oceanic plates
get “pulled down” into the mantle, melts, is recycled
 This process =Subduction
 Trenches are also called subduction zones
 Plate boundaries are violent (if slow) collisions.
•The plates colliding can be oceanic cont.
• Ocean plates always sink below, denser
•Produces earthquakes and volcanic mountain
e.x. Andes, Sierra Nevada
oceanic cont
oceanic  oceanic
Either plate could have dipped below the other, in this case.
•The plates colliding can be oceanic  oceanic
Earthquakes and volcanic island arcs = volcanic
island chain that follow trench curvature
Ex: Aleutian Islands, Mariana Islands
continental   continental
•The plates colliding can be
cont.   cont.
• Neither plate sinks, instead they
•Producing huge mountain ranges
Ex: Himalayas.
Fig. 2.14
Shear boundary
 Another type is shear
boundary or transform
 The plates slide past
each other
 Causes earthquakes
 Ex: San Andreas Fault,
Fig. 2.15
Two forces move the plates:
1) Slab-Pull theory - the sinking plate pulls the rest
behind it
2) Convection theory – the swirling mantle moves
the plate
Geological History of the Earth
Continental Drift and the Changing Oceans
 200 mil years ago all the continents were joined in
It was surrounded by a single ocean called Panthalassa
180 mil years ago a rift formed splitting it into two
large continents
Laurasia – North America and Eurasia
Gondwana – South America, Africa, Antarctica, India,
and Australia
Fig. 2.16
The plates are still
moving today.
Atlantic Ocean is
growing, Pacific is
C. Geological History of the Earth
2. The Record in the Sediments
 Studying sediments deposited in past, can learn about
the history of the planet
 2 types of marine sediments:
1) Lithogenous –
2) Biogenous –
 Mostly composed of calcium carbonate or silica
 Microfossils tell what organisms lived in the past
= animal-like
= animal-like
Oceans and Climate in the Past
 Past climate on Earth can be
determined by:
 Chemical composition of
Measure ratios of Mg and
Oxygen isotope ratios
Sr and Ca ratios in ancient
coral skeletons
Ice cores
•Fossil Agatized Coral is
Florida's state stone.
•28-25 million years ago
Fig. 2.18
C. Geological History of the Earth
3. Climate and Changes in Sea Level
 The Earth alternates between interglacial (warm) period
and ice age (cold) periods
 Sea level falls during ice ages because water is trapped in
glaciers on the continents
 Currently in an interglacial period
 Pleistocene Epoch , 2 mya, began last ice age…Peak was
18,000 yrs. ago
The Geological Provinces of the Ocean
 2 main regions of the sea floor.
 1)
 2)
A. Continental Margins
= Boundaries between the continental and oceanic crusts
Consists of:
A. Continental Margins
•The shallowest part
• Only 8% of the sea
floor, but biologically
rich and diverse
•Large submarine
canyons can be found
here, from past glaciation
• Ends at the shelf break,
where it steeply slopes
•Shelf 1km to 460 km (280 m)
A. Continental Margins
•The “edge” of the continent
• Slopes down from the shelf break to the deep-sea floor
•Submarine canyons
can carry sediments
from the shelf to the
sea floor.
•Reaches sea floor
at 10,000-16,500 ft
A submarine canyon
A. Continental Margins
•Some, similar to a river delta = deep-sea fan
A. Continental Margins
4. Active and Passive Margins
•Active margin = the
subducting plate
creates a trench
A. Continental Margins
4. Active and Passive Margins
•Passive margin –
no plate boundary
Ex: see next slide
Passive Margins Example: Atlantic Coast of U.S.A
•Buildup of sediments
•Broad coastal plains
•Barrier Islands
•Salt Marshes
Studying near continents
 Most of what Marine Biologists study are near
continents…why do you think?
B. Deep-Ocean Basins
 Most of sea floor , 10,000-16,500 ft
 Abyssal plain - flat region of the sea floor, but has
 A
 Seamounts – submarine volcanoes
 Guyots (“gee-oh”) –
 Trenches , subduction zones, = the deepest part of the
 Mariana Trench is 36,163 ft deep (11,022 m) the
deepest on Earth
C. Mid-Ocean Ridge and Hydrothermal Vents
 At the center of the
ridge, where the plates
pull apart =
 Water seeps down
through cracks, gets
heated by the mantle
 Then emerges through
hydrothermal vents
 350oC (660oF)
Fig. 2.26
C. Mid-Ocean Ridge and
Hydrothermal Vents
 Dissolved minerals from
the mantle, like sulfides,
are brought up
 Black smokers form
when minerals solidify
around a vent
 Marine life, including
chemosynthesizers, exist
around hydrothermal
Fig. 2.27
 Chimney-like
structures that
build up around
vents as the
minerals solidify.
Hawaiian Islands
 Part of the Emperor Seamount chain
 Made from a Hotspot =
 Pacific plate, slowly moving over the stationary
 Much debate still, a stationary hotspot or various
cracks in the crust.
Page 36
Grattan Seamount
 In S.Atlantic, following
the Mid-Atlantic Ridge
between the remote
islands of St. Helena
and Ascension.
 <iframe width="560" height="315"
src="" frameborder="0"
Tab. 2.1
 Helpful table for studying
Geology and Marine Biology
 What does geology have to do with marine
 Profoundly influences habitats= The natural environment where
organisms live
Sculpts shorelines
Determines water depth
Controls if muddy, sandy, rocky bottom
Creates new islands, ridges, mountains for organisms to colonize

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